1. Field of the Invention
Generally, the present disclosure relates to the manufacturing of sophisticated semiconductor devices, and, more specifically, to a compact RRAM (Resistance Random Access Memory) device with an embedded bipolar junction transistor selector structure and various methods of making such an RRAM device.
2. Description of the Related Art
As is well known, non-volatile memory devices are characterized in that there is no loss of data stored in their memory cells, even when an external power supply is removed. For that reason, such non-volatile memory devices are widely employed in a computer, a mobile communication system, a memory card and the like.
Flash memory structures are widely used in such non-volatile memory applications. The typical flash memory device employs memory cells having a stacked gate structure. The stacked gate structure typically includes a tunnel oxide layer, a floating gate, an inter-gate dielectric layer and a control gate electrode, which are sequentially stacked above a channel region. While flash memory structures have enjoyed enormous success, the continued and ever-present drive to reduce the size of integrated circuit products has created many challenges for the continued scaling of flash memory devices. Such challenges include scaling of program/erase voltages, access speed, reliability, the number of charges stored per floating gate, etc.
A resistance random access memory (RRAM) device is a memory device in which a variable resistance material layer is used as a data storage material layer. The resistance of the variable resistance material layer may be varied or changed based upon the polarity and/or amplitude of an applied electric pulse. The electric field strength or electric current density from the pulse, or pulses, is sufficient to switch the physical state of the materials so as to modify the properties of the material and establish a highly localized conductive filament (CF) in the variable resistance material. The pulse is of low enough energy so as not to destroy, or significantly damage, the material. Multiple pulses may be applied to the material to produce incremental changes in properties of the material. One of the properties that can be changed is the resistance of the material. The change may be at least partially reversible using pulses of opposite polarity or pulses having a different amplitude from those used to induce the initial change.
In general, after an RRAM device is initially fabricated, the variable resistance material layer does not exhibit any switching properties. Rather, a high-voltage, high-current process, a so-called FORMING process, is performed to initially form the localized conductive filament with oxygen vacancies from the cathode, establishing a low-resistance state (LRS) exhibiting a relatively high current flow. A so-called RESET process is performed to break the conductive filament and establish a high-resistance state (HRS) exhibiting a relatively low current flow. Note that the RESET process removes only a portion of the entire length of the conductive filament, i.e., the RESET process does not remove the entire conductive filament. After a RESET process is performed, a so-called SET process is performed to re-establish the conductive filament and thus the low-resistance state of the RRAM device. The SET process is essentially the same as the FORMING process except that the SET process is performed at a lower voltage than the FORMING process since the filament length to be re-established is shorter than the length of the conductive filament that was formed during the FORMING process.
The variable resistance material layer employed in an RRAM device may be comprised of a material capable of having its resistivity changed in response to an electrical signal. Such materials may include a perovskite material, such as a colossal magnetoresistive (CMR) material or a high temperature superconducting (HTSC) material, for example Pr.0.7 Ca.0.3 MnO3 (PCMO). Another example of a suitable material is Gd0.7CaO0.3BaCo2O5+5. Other possible materials for the variable resistance layer include transition metal oxides such as hafnium oxide, titanium oxide, nickel oxide, tungsten oxide, tantalum oxide, copper oxide, etc., manganites, titanates (e.g., STO:Cr), zirconates (e.g., SZO:Cr, Ca2Nb2O7:Cr, Ta2O5:Cr), and high Tc superconductors (e.g., YBCO). RRAM devices may be advantageous in highly scaled, high integration applications due to their relatively smaller foot print as compared to a capacitor based memory device in which memory characteristics are proportional to the size of a cell area.
The present disclosure is directed to a novel, compact RRAM (Resistance Random Access Memory) device structure and various methods of making such an RRAM device.